This invention relates generally to heat transfer devices and air filtration devices, and in particular to heat exchangers, ventilators, and enthalpy exchangers along with air filters. The invention is particularly well-suited for air-to-air regenerative heat exchangers utilizing high efficiency particulate air (HEPA) filter material as the regenerative heat exchanger.
Many individuals suffer from respiratory disorders, including allergies and asthma. In recent decades, scientists have known that poor quality indoor air seriously impacts human health. (American Lung Association, Washington, D.C. 20036, "Residential Air cleaning Devices: Types, Effectiveness and Health Impact", 1997) Reduction of indoor pollution helps alleviate the suffering of these individuals. Efforts to reduce indoor air pollution have been directed to three areas: ventilation, air cleaning and source control. The problem of providing adequate indoor ventilation is well known.
Modern energy efficient construction employs air "tight" structures to restrict infiltration of outside air. Lack of infiltration or natural ventilation has resulted in inadequate indoor ventilation. Standard 62-1989 of American Society of Heating, Refrigeration, and Air conditioning Engineers Inc. (ASHRAE), Atlanta, Georgia, states, "When infiltration and natural ventilation are insufficient to meet ventilation air requirements, mechanical ventilation shall be provided. The use of energy recovery ventilation systems should be considered for energy conservation purposes in meeting ventilation requirements." (Sec. 5.1.) Standard 62-1989 suggests 0.35 air changes per hour of continuous fresh air for living areas, but not less than 15 Cubic Feet per Minute (CFM) per person based on design occupancy. For a 2500 square-foot home, this equates to about 120 CFM.
Bringing outside air into a structure for ventilation purpose can be problematic and expensive. Utilizing gas or electric heat to preheat separate ventilation air in winter is inefficient. For example, if the outside air is 20.degree. C. colder than indoors, approximately 1.2 kW of heat is required to preheat the 120 CFM of required ventilation for a 2500 square-foot home. Use of a heat recovery ventilator is by far the most efficient way to ventilate, exchanging as much as 85% of the heat from warm (inside) exhaust air with the cool fresh air. In summer, use of a heat recovery ventilator also reduces air-conditioning load by exchanging cool dry exhaust air with warm humid fresh air. An "enthalpy" exchanger has been found to be particularly effective in humid climates.
Some prior art air-to-air heat exchanger technology for home use utilize a cross-flow heat exchanger core, e.g., Lifebreath.TM. heat recovery ventilator by Nutech Energy Systems, Inc. of London, Ontario, Canada; TherMax TW Model room ventilators made by Thermax Energy Recycling Ventilation Systems, Division of Kooltronic, Inc. of Hopewell, N.J.; NewAire.TM. air-to-air heat exchange ventilators made by Altech Energy of Madison, Wis.; U.S. Pat. No. 4,512,392 (Van Ee et al.) and U.S. Pat. No. 5,273,105 (Drake). A disadvantage of these devices is low heat exchanger effectiveness. The best theoretical effectiveness is approximately 70% for a cross-flow core. Practically, these devices only achieve a fraction of that effectiveness.
Other prior art technology includes the use of a rotary heat recovery, wheel, e.g., Honeywell "Perfect Window" System energy recovery ventilator, available from Honeywell, Inc. of Golden Valley, Minnesota. This device employs a rotating regenerative wheel, as well as a fresh air filter and a room air filter. Two types of rotary heat recovery wheels may be used-a desiccant wheel to transfer moisture and also dry heat, or a sensible wheel to transfer only dry heat. (However, as is known in the art, the sensible wheel will transfer moisture when the air drops below the dew point temperature as the air passes through the regenerative wheel.) An advantage of this technology is that high heat exchanger effectiveness is possible. A disadvantage is that it requires an additional moving part, i.e., the regenerative wheel. This regenerative wheel (rotary heat recovery wheel) is approximately 16 inches in diameter for one model. It rotates at about 30 RPM. On one side of the wheel there is outside air. On the other side, there is indoor air. A brush seal is used around the rim of the wheel, and in freezing conditions, warm moist air flowing past the seal will condense and freeze forming frost. If the frost melts, it may migrate to the rim of the wheel and refreeze which can cause the wheel to freeze up. To prevent wheel freeze up, an electric preheater on the incoming air is used to warm the air to 5.degree. F.(-15.degree. C.).
Yet other prior art technology which uses fixed, rotating or reciprocating heat exchanging beds or some method of periodically changing the airflow direction includes U.S. Pat. No. 3,978,912 (Penney et al.); U.S. Pat. No. 4,049,404 (Johnson); U.S. Pat. No. 4,391,321 (Thunberg); U.S. Pat. No. 4,493,366 (Ekman); U.S. Pat. No. 4,589,476 (Berner); U.S. Pat. No. 4,665,805 (Ekman); U.S. Pat. No. 4,688,626 (Tengesdal); U.S. Pat. No. 4,744,409 (Berner); U.S. Pat. No. 4,754,806 (Astle); U.S. Pat. No. 4,815,522 (Thunberg); U.S. Pat. No. 4,952,283 (Besik); U.S. Pat. No. 5,002,116 (Hoagland et al.); U.S. Pat. No. 5,050,667 (Berner et al.); U.S. Pat. No. 5,375,649 (Nilsen et al.) and D. A. Reay, "Heat Recovery Systems"(E.& F.N. Spoon, London, UK, 1979, pp. 17-35).
Another problem with bringing in ventilation air concerns the quality of the fresh air introduced into the room or structure. In many places, allergens, such as, pollen or mold spores, and/or other particulates, such as, soot from vehicle exhaust or emissions from industrial sites, exist in the outside fresh air through much of the year. Thus source control of air-borne pollutants, e.g., controlling the source of the allergens and/or particulates from the incoming fresh air, is important. Filtering these allergens and/or particulates out of the incoming fresh ventilation air is important for individuals subject to respiratory diseases, including severe allergy sufferers or asthma sufferers.
Filtering of the indoor air and trapping of pollutants, particulates and/or allergens generated in the indoor air is also important, since these too can create further respiratory distress. The indoor air generated pollutants, include, but are not limited to, cigarette smoke, pipe smoke, cigar smoke, smoke from the fireplace, organic pollutants, such as gasses from building materials, e.g. particle board, plywood, rugs, paints, varnishes, adhesives, or from cleaning supplies, personal care items, room deodorants, as well as other gases, such as radon, combustion products produced by unvented cooking and heating appliances, and particulates or allergens, such as, but not limited to, animal dander, dust mites, their feces and body parts, insect body parts, indoor molds and fungus, bacteria and viruses, etc.
Air cleaning devices which remove pollutants, allergens and particulates of a certain sizes are shown in the disclosure in the American Lung Association, Washington, D.C. 20036, "Residential Air cleaning Devices: Types, Effectiveness and Health Impact, pages 9-16, 1997, the disclosure of which is hereby incorporated by reference. This publication discloses that air cleaning devices can be tabletop/console units, portable room air cleaners or central filtration units. The air cleaning devices use mechanical filters, electronic filters, hybrid filters (mechanical/electrostatic) filters, gas phase filters or ozone generators. The mechanical filter is typically a flat filter, a pleated filter, or a High Efficiency Particulate Air filter, having the acronym HEPA.
Use of a pleated filter is also known in the medical airway ventilator art for use as a heat and moisture exchanger, see, PALL.TM. HME BB100F, PALL BIOMEDICAL, INC., Fajaido, PR. Here a maximum 24 hour usage is recommended. The filter is alleged to have "greater than 99.999% Bacterial/Viral removal Efficiency."
The HEPA filter technology is a known technology, see U.S. Pat. No. 4,629,482 to Davis and U.S. Pat. No. 4,685,944 to Allen et al. The "A" in the acronym HEPA is alternatively referred to as air, aerosol or arrestor. Thus a HEPA filter could be referred to as a "High Efficiency Particle Air" filter, a "High Efficiency Particle Aerosol" filter or a "High Efficiency Particle Arrestor" filter. The materials used for HEPA filters are typically glass fiber, glass-asbestos fiber, or other equivalent inorganic material and may include an organic binder material. The description of the HEPA filter unit, filter properties and testing are disclosed in the publications "Underwriters Laboratories, Inc., "Test Performance of High Efficiency Particulate Air Filter Units", UL 586 (1977), pp. 5-9, International Atomic Energy Agency, Vienna(IAEA), 1970, "Air Filters for Use at Nuclear Facilities, Technical Report Series No. 122, pp. 16-42, the disclosures of which are hereby incorporated by reference. The disclosure of accordian type, V-shaped pleated HEPA filter, having closely spaced pleats and surrounded by a rectangular frame or casing on one or both edges is known. (see, IAEA Rept.122(supra), pp. 16-17, and US Army Corps of Engineers, USACERL Technical Report (TR) FE-95/10, "Air Cleaning Systems and Indoor Air Quality: A Review", pp. 69-70, 1995).
In the conventional HEPA filter art, the filter(filter unit)/filter material is typically defined by the testing standards used in the filter's certification. Interestingly, the testing standards are not identical testing methods. For example, the terms "true" HEPA and "ASHRAE" HEPA are commonly used in the HEPA filter art. The definition of each of these types of HEPA filters is governed by a different measurement standard. As used throughout herein, "true" HEPA filter and "true" HEPA filter material means a high efficiency particle air filter(filter material) which removes at least 99.97% of 0.3 micron dioctylphthalate (DOP) particles as measured by MIL-STD-282, Method 102.9.1, May 28, 1956 (Military Standard Filter Units, Protective Clothing, Gas-Mask Components and Related Products: Performance-Test Methods, U.S. Government Printing Office, Washington, D.C., pp. 33-38 and FIG. 9), the disclosure of which is hereby incorporated by reference. In the art, the particle removal referred to in MIL-STD-282 Method 102.9.1, May 28, 1956, or equivalent federal standards, is frequently shortened to "removal" or "capture" of "99.97% of all 0.3 micron particles" or "particles having a particle diameter of 0.3 microns" or "remove 99.97% of airborne particulate matter of 0.3 microns or greater", or "remove a minimum of 99.97% of the particles having a size of 0.3 microns or greater" (see, U.S. Pat. No. 4,629,482 and 4,685,944). This convention is also used herein throughout, when referring to the true HEPA filter. As used therein throughout, "ASHRAE" HEPA filter and "ASHRAE" HEPA filter material means a high efficiency particle air filter (filter material) rated at least 85% (e.g. 85% or higher) Dust-Spot Efficiency percentage as measured by ASHRAE Standard 52.1-1992, including appendix A, "Gravimetric and Dust-Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter", hereinafter "Dust-Spot Procedure", American Society of Heating, Refrigerating and AirConditioning Engineers, Inc., Atlanta, Ga., (pages 1-32) the disclosure of which is hereby incorporated by reference. Since the measurement testing methods differ (e.g., "DOP test procedure of MIL-STD 282" versus "Dust-Spot Procedure" for particle removal efficiency ), it is difficult to do a direct comparison of filter efficiency of the two types of HEPA filters/filter materials. However, Table 3 of USACERL TR FE-95/10(supra), page 24, the disclosure of which is hereby incorporated by reference, shows that an approximate rating of 80% to 98% ASHRAE Dust-Spot Test (Dust-Spot Efficiency Percentage) filter generally removes 35% to 80% of 0.3 micron DOP particles.
The use of these different standards, as well as equivalent standards and/or differing standards and specifications are known in the HEPA filter manufacturing art, for example HEPA filters (filter units) commercially available from HEPA Corporation of Anaheim, Calif. meet various standards and specifications, such as, Fed. Std. 209, U.L. 900 class 1 and 2, Mil-F-51068, ASHRAE 52-76, MIL. STD. 282, U.L. 586 and IES-RP-CC-001-86.
Of particular interest is the HEPA filter unit commercially available as "3282 media" from Columbus Industries, Ashville, Ohio. This HEPA filter unit uses a HEPA grade mini-pleat material (media) made of glass micro fiber, e.g., submicron glass fibers with some synthetic fiber. This HEPA filter material captures 99.97% of all particles of0.3 micron in diameter passing through it. The media has been tested by the manufacturer and is true HEPA up to approximately 7 feet per minute (FPM) media velocity. The traversely pleated (accordion style pleating) material is surrounded on four sides of its periphery with a chipboard frame. The V-spaced pleated sides, the top and the bottom of the filter unit are glued to a frame side, thus ensuring sealing around the frame perimeter. The frame has a frame opening on each of two opposed sides exposing the edges of the pleats. The mini-pleating is created by scoring the strip of HEPA filter material at specific size intervals, applying a glue bead separator on both sides of the strip at the location of the scoring, and accumulating the HEPA filter material, e.g., stacking the pleats into a finished accordian style. The use of the glue bead separator allows the pleating of the filter material to remain at a fixed pleat density, e.g., pleats per inch, once the glue cools and solidifies.
Where an air cleaning device is used within a room, it acts to clean the air by removing particulates, pollutants and allergens. When the air cleaning device intercepts fresh air bearing particulates, pollutants and allergens, prior to dispersal into the room, it filters the fresh air of these materials, providing source control of the particulates, pollutants and allergens. This important function prevents mixing of the particulate, pollutants and allergens introduced in the fresh air with the indoor air.
The HEPA filter is also known to be useful in the removal of radioactive or biologically hazardous materials particles from contaminated air before this air is exhausted to the atmosphere, see, U.S. Pat. No. 4,685,944, issued to Allan et. al. The HEPA filter material traps the particles in the airstream passing through the HEPA filter. The flow of air through this filter is unidirectional, thus large dust particles trapped on the HEPA filter easily impede the unidirectional air flow, causing the HEPA filter to plug up, requiring filter replacement.
Most of the present air ventilation/heat recovery technology are large, heavy, bulky devices which are expensive, difficult to install, and complex, sometimes requiring preheating incoming cold air. Whereas the, standard console HEPA air cleaning devices utilizing a HEPA filter therein, such as, the HONEYWELL HEPA/CPZ.TM. air cleaner and the HONEYWELL ENVIRACAIRE.TM. air cleaner with HEPA filters, are small portable devices, which filter only the indoor air in the room. These HONEYWELL.TM. console air cleaning devices are commercially available, for example, from Allergy Asthma Technology Ltd., Morton Grove, Ill. These small portable devices, likewise only have unidirectional flow, with the attendant problem of buildup of large dust particles impeding the unidirectional flow through the HEPA filter.
As, used herein this application, "HEPA" filter and/or "HEPA" filter material includes the true HEPA filter and/or true HEPA filter material (or a filter and/or filter material tested by methods equivalent to MIL-STD-282, Method 102.9.1), as well as, the ASHRAE HEPA filter and/or ASHRAE HEPA filter material (or a filter and/or filter material tested by methods equivalent to ASHRAE Standard 52.1-1992).
Typically, to provide the air cleaning, source control and ventilation at least two units/devices are needed. This becomes expensive in terms of costs of the devices and maintance of the devices. Thus what is needed is a low cost device which provides incoming ventilation air which is both HEPA filtered and HEPA conditioned by heat and moisture exchange. Not withstanding the many known practical design problems for air-to-air heat exchangers with air filtration, the art has not responded to date with the production of a compact, lighter weight, air-to-air heat recovery ventilator using a regenerative heat exchanger made of pleated HEPA filter material and not requiring any heater to heat incoming air to avoid freeze-up problems in the heat recovery ventilator, and also not requiring complex rotating seals in the regenerative heat exchanger between an indoor climate and an outdoor climate.